The present invention relates to a resin composition having an excellent sliding property and more particularly to a resin composition having a sliding property and employable under severe conditions in which a mating material is a metal and the resin composition is used in a part sliding at high speed under a high load (the temperature of the said part inevitably rises to a high level due to frictional heat). The present invention is also concerned with an improved sealing member comprising the said resin composition and used in a scroll type compressor or a vacuumpump.
Recently, various synthetic resin products have come to be used as mechanical parts for reducing the weight of machine and product cost. Also as parts for which sliding property is required, synthetic resin products are now utilized frequently.
Heretofore, as resin compositions superior in sliding property there have been used those comprising resins such as polyamide, polyacetal, polyphenylene sulfide and polytetrafluoroethylene resins, and an additive for improving the sliding characteristic such as a solid lubricant or a lubricating oil.
Such conventional resin compositions are employable without causing any special problem in the region of relatively low load and speed, but as the load and speed increase, they become easier to be worn out, or may seize or melt to an unemployable extent due to frictional heat. For the purpose of improving the abrasion resistance and heat resistance there are used glass fibers, carbon fibers and various whiskers. But these fillers involve a problem of causing wear of metal or resin of a mating member used in a sliding portion.
More particularly, sliding characteristics required for sliding portions which slide at high load and speed are physical properties, including abrasion resistance and heat resistance, rather than coefficient of friction, unlike the prior art, so it is necessary to select materials from a standpoint entirely different from the conventional standpoint on sliding portions.
Among various plastic materials, high melting, thermotropic, liquid crystalline polymers are high in the resistance to heat, so do not seize or melt due to frictional heat.
Usually, a mating member in a sliding part is constituted by a metallic material such as aluminum alloy or steel. Since thermotropic liquid crystalline polymers per se are inferior in the abrasion resistance in comparison with such metals, so have not been used in such sliding portions as mentioned above.
The present inventors found out that a so-called glassy carbon characterized by its particle fracture surface having a glassy luster is effective as an additive capable of solving the above-mentioned problems involved in thermotropic liquid crystalline polymers. However, such glassy carbon sometimes contains an incompletely carbonized or uncarbonized substance because it is produced by carbonizing a thermosetting resin such as a phenolic resin.
Even in the case of a glassy carbon which has been identified by having a glassy luster at a fracture surface thereof or by having a wide peak at a specific diffraction angle in an X-ray diffraction pattern thereof, it sometimes contains an incompletely carbonized or uncarbonized substance.
In the case of incorporating a glassy carbon into a thermoplastic resin, the presence of such incompletely carbonized or uncarbonized substance usually causes no problem in the molding step of the thermoplastic resin or in the resulting molded product. This is because such incompletely carbonized or uncarbonized substance is considered to be the starting thermosetting resin itself of the glassy carbon, or a thermally decomposed oligomer thereof.
One feature of the aforementioned thermotropic liquid crystalline polymers resides in their melting point which is very high even to an abnormal extent, and hence their molding temperature is extremely high. In this case, the foregoing incompletely carbonized or uncarbonized substance contained in a glassy carbon is apt to decompose and evolve gas. Particularly in molding at high temperature and pressure such as compression or injection molding, the evolution of gas, even in a very small amount, causes a serious defect in the molding process or in the resulting molded product. In the extreme case, it is difficult to effect the molding itself due to the evolution of gas, or blister may be recognized on the surface of the resulting molded product, thus leading to the loss of commercial value of the product.
In a scroll compressor, a pair of scroll members are arranged in mesh with each other, one scroll being fixed and the other being allowed to revolve along a certain radius, and the space surrounded by both scroll members is gradually contracted toward the center to compress gas. In a scroll type vacuum pump, the relative rotation between the fixed and movable scrolls is in a reverse relation as compared with that in the compressor. Usually, as the material of these scroll members there is used a metallic material such as aluminum alloy or steel.
In the scroll type compressor or vacuum pump, it is very important to attain a good sealing property of the sliding portion between both scroll members, and to this end a sealing member is fitted in the end faces of the scroll members.
Therefore, such sealing member is required to have not only sealing property and sliding property such as abrasion resistance but also resistance to heat, creep and chemicals. In other words, extremely strict conditions are required as compared with ordinary sliding conditions.
Heretofore, as such sealing member there has been mainly used a sealing member comprising polytetrafluoroethylene resin and fillers. However, a composition containing polytetrafluoroethylene resin as a main component cannot be subjected to injection molding, and a desired product is obtained, for example by punching a compression-molded flat plate, so the productivity is poor and the product cost is inevitably high.
Compositions containing aromatic polyether ketone resins, polyallylene sulfide resins, or polyether imide resins, as main components, can be subjected to injection molding, but are still unsatisfactory in point of heat resistance at a high temperature during the operation of a compressor or a vacuum pump and the resistance to fron gas used as a refrigerant in the compressor.
The vortical sealing member used in a scroll type compressor or vacuum pump is small in sectional area and long, as shown in FIG. 1, so the injection molding using a conventional resin is difficult to be performed because the fluidity of the resin is not satisfactory. Also in point of sliding property, conventional resins have been unsatisfactory.
The present invention has been accomplished for overcoming the above-mentioned problems and it is the object of the invention to provide a resin composition for sliding movement superior in abrasion resistance, having heat resistance, easy to be molded, and capable of affording molded products of high commercial value, and also provide a sealing member in a scroll type compressor or vacuum pump, superior in the resistance to heat and to fron gas, capable of being subjected to injection molding, and further superior in productivity.